插件核心思想
1.aapt的改造
分别对不同的插件项目分配不同的packageId，然后对各个插件的资源进行编译，生成R文件，然后与宿主项目的R文件进行id的合并。
要求：由于最终会将所有的资源文件id进行合并，因此，所有的资源名称均不能相同。
2.运行ClassLoader加载各Bundle
和MultiDex的思路是一样的，所有的插件都被加载到同一个ClassLoader当中，因此，不同插件中的Class必须保持包名和类名的唯一。否则，加载过的类不会再次被加载。
优缺点：各个Bundle之间完全可以相互调用，但是这也造成了各个Bundle之间ClassLoader的非隔离性。并且随着数组的加长，每次findClass的时间会变长，对性能照成一定长度的影响。
让我们在熟悉一下这张图：
在DynamicAPK框架中，每个Bundle被加载到ClassLoader的调用栈如下：
Bundle的Application：BundleBaseApplication
->BundleBaseApplication(onCreate)
->BundleCore(run)
->BundleImpl(optDexFile)
->BundleArchiveRevision(optDexFile)
->BundlePathLoader(installBundleDexs)
->...
如下图所示：
3.热修复
由于所有的插件都被加载到同一个ClassLoader当中，因为，热修复的方案都是从dexElements数组的顺序入手，修改expandFieldArray方法的实现，将修复的类放到dexElements的前方。核心代码如下（详见BundlePathLoader）：
private static void expandFieldArray(Object instance, String fieldName,
Object[] extraElements,boolean isHotFix) throws NoSuchFieldException, IllegalArgumentException,
IllegalAccessException {
synchronized (BundlePathLoader.class) {
Field jlrField = findField(instance, fieldName);
Object[] original = (Object[]) jlrField.get(instance);
Object[] combined = (Object[]) Array.newInstance(
original.getClass().getComponentType(), original.length + extraElements.length);
if(isHotFix) {
System.arraycopy(extraElements, 0, combined, 0, extraElements.length);
System.arraycopy(original, 0, combined, extraElements.length, original.length);
}else {
System.arraycopy(original, 0, combined, 0, original.length);
System.arraycopy(extraElements, 0, combined, original.length, extraElements.length);
}
jlrField.set(instance, combined);
}
}
调用的关键代码如下(HotPatchItem.class)：
public void optDexFile() throws Exception{
List<File> files = new ArrayList<File>();
files.add(this.hotFixFile);
BundlePathLoader.installBundleDexs(RuntimeArgs.androidApplication.getClassLoader(), storageDir, files, false);
}
public void optHotFixDexFile() throws Exception{
List<File> files = new ArrayList<File>();
files.add(this.hotFixFile);
BundlePathLoader.installBundleDexs(RuntimeArgs.androidApplication.getClassLoader(), storageDir, files, true);
}4.运行时资源的加载
所有插件的资源都加载到DelegateResources中，关键代码如下：
DelegateResources.class
...
public static void newDelegateResources(Application application, Resources resources) throws Exception {
List<Bundle> bundles = Framework.getBundles();
if (bundles != null && !bundles.isEmpty()) {
Resources delegateResources;
List<String> arrayList = new ArrayList();
arrayList.add(application.getApplicationInfo().sourceDir);
for (Bundle bundle : bundles) {
arrayList.add(((BundleImpl) bundle).getArchive().getArchiveFile().getAbsolutePath());
}
AssetManager assetManager = AssetManager.class.newInstance();
for (String str : arrayList) {
SysHacks.AssetManager_addAssetPath.invoke(assetManager, str);
}
//处理小米UI资源
if (resources == null || !resources.getClass().getName().equals("android.content.res.MiuiResources")) {
delegateResources = new DelegateResources(assetManager, resources);
} else {
Constructor declaredConstructor = Class.forName("android.content.res.MiuiResources").getDeclaredConstructor(new Class[]{AssetManager.class, DisplayMetrics.class, Configuration.class});
declaredConstructor.setAccessible(true);
delegateResources = (Resources) declaredConstructor.newInstance(new Object[]{assetManager, resources.getDisplayMetrics(), resources.getConfiguration()});
}
RuntimeArgs.delegateResources = delegateResources;
AndroidHack.injectResources(application, delegateResources);
StringBuffer stringBuffer = new StringBuffer();
stringBuffer.append("newDelegateResources [");
for (int i = 0; i < arrayList.size(); i++) {
if (i > 0) {
stringBuffer.append(",");
}
stringBuffer.append(arrayList.get(i));
}
stringBuffer.append("]");
log.log(stringBuffer.toString(), Logger.LogLevel.DBUG);
}
}
...
上述代码就是将所有Bundle中的资源，通过调用AssetManager的addAssetPath方法，加载到assetManager对象中，然后再用assetManager对象，创建delegateResources对象，并保存在RuntimeArgs.delegateResources当中，然后调用AndroidHack.injectResources方法，对Application和LoadedApk中的mResources成员变量进行注入，代码如下：
public static void injectResources(Application application, Resources resources) throws Exception {
Object activityThread = getActivityThread();
if (activityThread == null) {
throw new Exception("Failed to get ActivityThread.sCurrentActivityThread");
}
Object loadedApk = getLoadedApk(activityThread, application.getPackageName());
if (loadedApk == null) {
throw new Exception("Failed to get ActivityThread.mLoadedApk");
}
SysHacks.LoadedApk_mResources.set(loadedApk, resources);
SysHacks.ContextImpl_mResources.set(application.getBaseContext(), resources);
SysHacks.ContextImpl_mTheme.set(application.getBaseContext(), null);
}其中，上述获取LoadedApk的代码，也是通过反射，获取运行时ActivityThread类的LoadedApk对象.
5.运行时动态替换Resource对象
ContextImplHook,动态替换getResources
为了控制startActivity的时候，能够及时替换Activity的Resource和AssetsManager对象，使用ContextImplHook类对Comtext进行替换，然后动态的返回上一步加载的RuntimeArgs.delegateResources委托资源对象。ContextImplHook的核心代码如下：
@Override
public Resources getResources() {
log.log("getResources is invoke", Logger.LogLevel.INFO);
return RuntimeArgs.delegateResources;
}
@Override
public AssetManager getAssets() {
log.log("getAssets is invoke", Logger.LogLevel.INFO);
return RuntimeArgs.delegateResources.getAssets();
}
如何在Activity跳转过程中，动态的替换呢
通过反射替换ActivityThread的mInstrumentation对象，替换成InstrumentationHook.class,然后就可以在执行startActivity时，拦截其newActivity和callActivityOnCreate方法，在newActivity方法中，动态的替换newActivity的mResources对象。在callActivityOnCreate方法中将ContextImplHook注入到新创建的Activity中。核心代码如下：
@Override
public Activity newActivity(Class<?> cls, Context context, IBinder iBinder, Application application, Intent intent, ActivityInfo activityInfo, CharSequence charSequence, Activity activity, String str, Object obj) throws InstantiationException, IllegalAccessException {
Activity newActivity = this.mBase.newActivity(cls, context, iBinder, application, intent, activityInfo, charSequence, activity, str, obj);
if (RuntimeArgs.androidApplication.getPackageName().equals(activityInfo.packageName) && SysHacks.ContextThemeWrapper_mResources != null) {
SysHacks.ContextThemeWrapper_mResources.set(newActivity, RuntimeArgs.delegateResources);
}
return newActivity;
}
@Override
public Activity newActivity(ClassLoader classLoader, String str, Intent intent) throws InstantiationException, IllegalAccessException, ClassNotFoundException {
Activity newActivity;
try {
newActivity = this.mBase.newActivity(classLoader, str, intent);
if (SysHacks.ContextThemeWrapper_mResources != null) {
SysHacks.ContextThemeWrapper_mResources.set(newActivity, RuntimeArgs.delegateResources);
}
} catch (ClassNotFoundException e) {
String property = Framework.getProperty("ctrip.android.bundle.welcome", "ctrip.android.view.home.CtripSplashActivity");
if (StringUtil.isEmpty(property)) {
throw e;
} else {
List runningTasks = ((ActivityManager) this.context.getSystemService(Context.ACTIVITY_SERVICE)).getRunningTasks(1);
if (runningTasks != null && runningTasks.size() > 0 && ((ActivityManager.RunningTaskInfo) runningTasks.get(0)).numActivities > 1) {
if (intent.getComponent() == null) {
intent.setClassName(this.context, str);
}
}
log.log("Could not find activity class: " + str, Logger.LogLevel.WARN);
log.log("Redirect to welcome activity: " + property, Logger.LogLevel.WARN);
newActivity = this.mBase.newActivity(classLoader, property, intent);
}
}
return newActivity;
}
@Override
public void callActivityOnCreate(Activity activity, Bundle bundle) {
if (RuntimeArgs.androidApplication.getPackageName().equals(activity.getPackageName())) {
ContextImplHook contextImplHook = new ContextImplHook(activity.getBaseContext());
if (!(SysHacks.ContextThemeWrapper_mBase == null || SysHacks.ContextThemeWrapper_mBase.getField() == null)) {
SysHacks.ContextThemeWrapper_mBase.set(activity, contextImplHook);
}
SysHacks.ContextWrapper_mBase.set(activity, contextImplHook);
}
this.mBase.callActivityOnCreate(activity, bundle);
}
总结如下图,Resource的加载和动态替换：
6.插件Activity在宿主AndroidManifest中的预注册
每个插件的Activity，必须在宿主的AndroidManifest.xml中进行注册。
DynamicAPK源码导读：
源代码的目录结构图
- framework
管理各个Bundle以及各个Bundle的封装、版本控制等。
- hack
通过反射的形式，hack类，方法，成员变量等
- hotpatch
热修复相关的封装
- loader
对MultiDex的修改，各Bundle加载到ClassLoader，热修复。
- log
日志管理
- runtime
运行时，对Resources进行动态替换
- util
工具类
参考
[1.AssetManager源码]()
2.LoadedApk源码
3.ActivityThread源码
4.DynamicAPK源码